1. Field of the Invention
This invention relates generally to nucleic acid sequences encoding polypeptides that are associated with growth and/or abiotic stress responses and/or abiotic stress tolerance in plants. In particular, this invention relates to nucleic acid sequences encoding polypeptides that increase plant growth under conditions of limited water availability and confer drought, cold, and/or salt tolerance to plants.
2. Background Art
Abiotic environmental stresses, such as drought stress, salinity stress, heat stress, and cold stress, are major limiting factors of plant growth and productivity. Crop losses and crop yield losses of major crops such as soybean, rice, maize (corn), cotton, and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries.
Plants are typically exposed during their life cycle to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on development, growth, and yield of most crop plants are profound. Continuous exposure to drought conditions causes major alterations in the plant metabolism which ultimately lead to cell death and consequently yield losses.
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies to develop new lines of plants that exhibit resistance (tolerance) to these types of stresses are relatively slow and require specific resistant lines for crossing with the desired line. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to drought, cold, and salt tolerance in model drought- and/or salt-tolerant plants are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways. This multi-component nature of stress tolerance has not only made breeding for tolerance largely unsuccessful, but has also limited the ability to genetically engineer stress tolerant plants using biotechnological methods.
Common damage from different stresses such as drought, salinity, and cold stress, appears to be mostly due to dehydration (Smirnoff, 1998, Curr. Opin. Biotech. 9:214-219). Drought (water stress)-tolerant and -sensitive plants can be clearly distinguished by the dramatic accumulation of ions and solutes in tolerant plants that leads to osmotic adjustments in the plants (Bohnert H. J and Jensen. R. G., 1996, TIBTECH 14:89-97). Drought and high salt conditions may correspond with mineral nutrition as a consequence of (1) reduced transport of ions through the soil to the roots; and/or (2) modified uptake of ions by the roots.
The SCARECROW (SCR) gene was identified in Arabidopsis and is expressed specifically in root progenitor tissues of plant embryos and in certain root and stem tissues. The SCR gene encodes a novel putative transcription factor and is required for asymmetric cell division in an Arabidopsis root. Modulation of SCR expression levels can be used to advantageously modify root and aerial structures of transgenic plants and enhance the agronomic properties of such plants. Mutation of the SCR gene results in a radial pattern defect and loss of a ground tissue layer in the root.
Pysh and co-workers identified a number of Arabidopsis expressed sequence tags (ESTs) that have similarity to the Arabidopsis SCR amino acid sequence and designated them the Scarecrow-like genes (SCL) (Pysh et al., 1999, Plant J. 18:111-119). The SCL genes comprise a novel gene family, referred to as the GRAS gene family, based on the locus designations of three genes: the gibberellin-acid insensitive (GAI) locus, the repressor of GAI (RGA) locus, and the scarecrow (SCR) locus. The GRAS/SCL gene products have been reported to be restricted to higher plants and are plant-specific proteins that participate in various developmental processes. Members of the GRAS/SCL family have a variable N-terminus and a highly conserved C-terminus that contains five recognizable motifs: the leucine heptad repeat I (LHR I), the VHIID motif, the leucine heptad repeat II (LHR II), the PFYRE motif, and the SAW motif.
The GRAS/SCL proteins function as transcription factors but are not restricted to their role in asymmetric cell division. For example, the PAT1 protein, has been shown to be involved in phytochrome A signal transduction of Arabidopsis thaliana (Bolle et al., Genes Dev., 2000, 14:1269-1278), and the tomato gene Lateral suppressor (Ls) functions in the formation of lateral branches. Two members of the GRAS family, the GAI and the RGA genes, play important roles in the gibberellin acid (GA) signal transduction pathway. Arabidopsis plants with a mutation at the GAI locus do not respond to exogenously applied GA and have a reduced stature (Koorneef et al., 1985, Physiol. Plant. 65:33-39). The SLR1 of rice has been identified as a GAI ortholog and has been demonstrated to be involved in the GA-signaling pathway in corn, rice, barley, grape, and wheat (Hynes et al., 2003, Transgenic Research 12:707-714). Overexpression of the Arabidopsis GAI in tobacco and rice produced a dwarf phenotype, as compared to a wild-type plant (Hynes et al., 2003, Transgenic Research 12:707-714).
There is a fundamental physiochemically-constrained trade-off, in all terrestrial photosynthetic organisms, between carbon dioxide (CO2) absorption and water loss (Taiz and Zeiger, 1991, Plant Physiology, Benjamin/Cummings Publishing Co., p. 94). CO2 needs to be in aqueous solution for the action of CO2 fixation enzymes such as Rubisco (Ribulose 1,5-bisphosphate Carboxylase/Oxygenase) and PEPC (Phosphoenolpyruvate carboxylase). As a wet cell surface is required for CO2 diffusion, evaporation will inevitably occur when the humidity is below 100% (Taiz and Zeiger, 1991, p. 257). Plants have numerous physiological mechanisms to reduce water loss (e.g. waxy cuticles, stomatal closure, leaf hairs, sunken stomatal pits). As these barriers do not discriminate between water and CO2 flux, these water conservation measures will also act to increase resistance to CO2 uptake (Kramer, 1983, Water Relations of Plants, Academic Press p. 305). Photosynthetic CO2 uptake is absolutely required for plant growth and biomass accumulation in photoautotrophic plants.
Water Use Efficiency (WUE) is a parameter frequently used to estimate the trade off between water consumption and CO2 uptake/growth (Kramer, 1983, Water Relations of Plants, Academic Press p. 405). WUE has been defined and measured in multiple ways. One approach is to calculate the ratio of whole plant dry weight, to the weight of water consumed by the plant throughout its life (Chu et al., 1992, Oecologia 89:580). Another variation is to use a shorter time interval when biomass accumulation and water use are measured (Mian et al., 1998, Crop Sci. 38:390). Another approach is to utilize measurements from restricted parts of the plant, for example, measuring only aerial growth and water use (Nienhuis et al 1994 Amer J Bot 81:943). WUE also has been defined as the ratio of CO2 uptake to water vapor loss from a leaf or portion of a leaf, often measured over a very short time period (e.g. seconds/minutes) (Kramer, 1983, p. 406). The ratio of 13C/12C fixed in plant tissue, and measured with an isotope ratio mass-spectrometer, also has been used to estimate WUE in plants using C3 photosynthesis (Martin et al., 1999, Crop Sci. 1775).
An increase in WUE is informative about the relatively improved efficiency of growth and water consumption, but this information taken alone does not indicate whether one of these two processes has changed or both have changed. In selecting traits for improving crops, an increase in WUE due to a decrease in water use, without a change in growth would have particular merit in an irrigated agricultural system where the water input costs were high. An increase in WUE driven mainly by an increase in growth without a corresponding jump in water use would have applicability to all agricultural systems. In many agricultural systems where water supply is not limiting, an increase in growth, even if it came at the expense of an increased water use (i.e. no change in WUE), could also increase yield. Therefore new methods to increase both WUE and biomass accumulation are required to improve agricultural productivity. As WUE integrates many physiological processes relating to primary metabolism and water use, it is typically a highly polygenic trait with a large genotype by environment interaction (Richards et al., 2002, Crop Sci. 42:111). For these and other reasons, few attempts to select for WUE changes in traditional breeding programs have been successful.
Although some genes that are involved in plant growth and/or stress responses in plants have been characterized, the characterization and cloning of plant genes that confer stress tolerance and/or increased growth under water-limited conditions remains largely incomplete and fragmented. For example, certain studies have indicated that drought and salt stress in some plants may be due to additive gene effects, in contrast to other research that indicates specific genes are transcriptionally activated in vegetative tissue of plants under osmotic stress conditions. Although it is generally assumed that stress-induced proteins have a role in stress tolerance, direct evidence is still lacking, and the functions of many stress-responsive genes are unknown.
There is a need, therefore, to identify additional genes expressed in stress tolerant plants and/or plants efficient in water use that have the capacity to confer stress resistance and or increased growth under water-limited conditions to the host plant and to other plant species. Newly generated stress tolerant plants and/or plants efficient in water use will have many advantages, such as increasing the range in which crop plants can be cultivated by, for example, decreasing the water requirements of a plant species. Plant and crop growth and yield is commonly limited by water availability. Increasing plant growth under conditions of limited water availability can increase crop yields in all the major global markets.